TEK 475: Dead on Start Up.


Redguuz
 

Hello Specialists,

Maybe you could give me some advice how to "debug" my broken TEK 475.

It had worked the hour before and I switched it off.
Realized that I might need it later, I repowered it again, but the Fan started and stopped immediately and the graticule lighting was off. No trace.

I have the ARTEK manuals and the Repair manuals of the US Military. (I have not opened the scope yet).
I concluded that at least the +15V Power Supply has failed but I understand (from the SM manual P172/240), that there is a "hierachy" of power supplies and that, if a PS "higher" in the hierarchy fails, it will influence the lower ones.
Apparently to protect circuits from unbalanced voltages. (is this correct?).

Questions;
1) I understand that I should start with checking the +50V (UNREG) and then 50V (REG)\, next the+ 110V, +15V, and then +5V, -8V and finally -15V.

Is this correct?.

3) Are the composed PS Buffer electrolytic capacitors still available as New Old Stock?
4) I understand that TEK used a way too low margin in Voltage rating for the Tantum capacitors on power rails.
- Are there any which particularly have a bad reputation and should ALWAYS be replaced?.
or
5) Should I eventually replace ALL those tantalum caps on the various power rails with 105 deg. C electrolytic capacitors?
- or are there "solid aluminum" alternatives to the tantalums used in that era (mid 1970's - mid 1980 's ?

Appreciate your advice, before I will be embarking on a repair attempt.

NB.
I have a HAMEG HM-605 (60 MHz) which I had to repair (broken Bridge rectifier (1 diode interrupted) in a -12V supply rail) , before I could trying to repair the TEK.
The HAMEG suits me well, but the TEK 475 has a much higher bandwidth (200 MHz).

So I like to resurrect the TEK 475.


Tom Lee
 

Here are some of my answers to your questions.

Re: order of checking voltages. The order in which you check doesn't matter, frankly. Just check 'em all. You merely want to assess what's broken, and in which way. Then take those clues and act on whatever they imply.

Common causes of what you're seeing include the caps you mention, but you should also check the bridges. I've seen more than a couple where one or two of the internal rectifiers was defective.

If you find a shorted tant (very common and, without any other information, I'd suspect that as the cause of your problem) and wish to replace it with another tant, Tek recommended that the replacement have a voltage rating that is at least 2x the nominal applied voltage. You could replace them with standard aluminum electrolytics, but be advised that they likely won't have the low ESR or ESL of tants. That difference can be important in some critical circuits. If you want to move away from tants (and I wouldn't blame you if you did), I'd recommend polymer-aluminum electrolytics. They've gotten good enough that I would be using them all the time where one would have previously selected a tantalum, except that I have a large quantity of tants.

Some folks recommend wholesale replacement of all caps, on the theory that one failure implies that the rest are likely to follow in the not too distant future, so you might as well reset the lifetime clock now, so to speak. I generally don't follow that practice, because the chance of killing something in the act of replacing any one cap is nonzero. Perhaps small, but nonzero nonetheless, so the probability of doing more bad than good becomes uncomfortably high when computing the overall proabilities. There are pros and cons of complete recaps, and how that resolves for me is very much vintage- and type-dependent, as well as on the skills of the person doing the work. For a 475, I would just replace what's dead, as it isn't all that onerous to open up. Others here will shortly chime in with their opinions.

I'm sure that you will be able to resurrect that fine scope without too much drama. Good luck with the repair!

-- Cheers
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/20/2021 01:50, Redguuz via groups.io wrote:
Hello Specialists,

Maybe you could give me some advice how to "debug" my broken TEK 475.

It had worked the hour before and I switched it off.
Realized that I might need it later, I repowered it again, but the Fan started and stopped immediately and the graticule lighting was off. No trace.

I have the ARTEK manuals and the Repair manuals of the US Military. (I have not opened the scope yet).
I concluded that at least the +15V Power Supply has failed but I understand (from the SM manual P172/240), that there is a "hierachy" of power supplies and that, if a PS "higher" in the hierarchy fails, it will influence the lower ones.
Apparently to protect circuits from unbalanced voltages. (is this correct?).

Questions;
1) I understand that I should start with checking the +50V (UNREG) and then 50V (REG)\, next the+ 110V, +15V, and then +5V, -8V and finally -15V.

Is this correct?.

3) Are the composed PS Buffer electrolytic capacitors still available as New Old Stock?
4) I understand that TEK used a way too low margin in Voltage rating for the Tantum capacitors on power rails.
- Are there any which particularly have a bad reputation and should ALWAYS be replaced?.
or
5) Should I eventually replace ALL those tantalum caps on the various power rails with 105 deg. C electrolytic capacitors?
- or are there "solid aluminum" alternatives to the tantalums used in that era (mid 1970's - mid 1980 's ?

Appreciate your advice, before I will be embarking on a repair attempt.

NB.
I have a HAMEG HM-605 (60 MHz) which I had to repair (broken Bridge rectifier (1 diode interrupted) in a -12V supply rail) , before I could trying to repair the TEK.
The HAMEG suits me well, but the TEK 475 has a much higher bandwidth (200 MHz).

So I like to resurrect the TEK 475.




Michael W. Lynch
 

Tom is spot on. I have repaired dozens of these scopes and do not subscribe to the "shotgun" approach of replacing all tantalum's. When they fail, tantalum caps usually do so in spectacular fashion and typically announce failure with a quantity of "magic smoke". I replace tantalum caps only if the related supplies or circuits exhibit symptoms that can be contributed to no other source. 2X voltage rating or more is best. That scheme has worked for me and I have seen relatively few failures. The Bulk filter caps are typical failure points so that is where you can spend your initial investigation. I have found that you can solder a good cap across the pads of the bulk filter cap of the offending supply and restore function as "proof" that the filter cap is the issue. it is possible that the bridge rectifier for the +15V circuit is bad, but that will become apparent upon testing of the bulk filters. Check ripple on all supplies while you are at it. and repair as needed. Good luck! ,

--
Michael Lynch
Dardanelle, AR


Mark Vincent
 

Replacing the filter cans is a good idea. I did it in mine using high temp and long life types. If you can get low ESR types for these in addition to the long life and temp., good. Raising the values is fine. The 1.5A bridge rectifiers should be 4A. If you have any that are higher in voltage rating of the original, 200V, fine. It will not hurt to put in a higher voltage type. The higher current is the main thing. Make sure to put jumpers between the negative pads if you use snap-in or radial leaded types as replacements. The output filters on the supplies can be raised in value, e.g. 10mfd to 47mfd. I use Nichicon ULD or UHE for these. The former has the lower ESR. The UHE is also a great choice. C1304 and C1374 I put in a film type. Decoupling types can be left in unless bad.

Check VR1416. If open, use BZX85B9V1.

Check all 151-0367-00 transistors. Any that are leaky C to E, use KSP10BU as replacements. The pinout of these is BEC.

Some resistors I replaced with a higher wattage because the originals were too small and the heat changed the resistance making it out of tolerance or open in some cases. R1378 is likely out tolerance. I used the Vishay VR37 series as the replacement. The symptom of this resistor being too high, about 28meg and higher, is the trace will be bright at turn on then dim down as it warms up over 15 minutes. I have seen this problem enough times.

Mark


Redguuz
 

Dear Tom, Michael and Mark,

Thank you very much for your extensive answers.
I will attempt to diagnose / repair the TEK475 in the next few weeks and will surely report back on the diagnosis/results.

I'm also very grateful for the advise of Mark on the Changing Brightness due to an OFF SPEC R1378.
This scope initially had this problem very much, but somehow it became less ( I bought it 4 years ago).
I will have a look at this circuit as well.

Cheers,

Martin


Steven Bender
 

1) I understand that I should start with checking the +50V (UNREG) and then 50V (REG)\, next the+ 110V, +15V, and then +5V, -8V and finally -15V.

Is this correct?.

4) I understand that TEK used a way too low margin in Voltage rating for the Tantum capacitors on power rails.
- Are there any which particularly have a bad reputation and should ALWAYS be replaced?.

or

5) Should I eventually replace ALL those tantalum caps on the various power rails with 105 deg. C electrolytic capacitors?
- or are there "solid aluminum" alternatives to the tantalums used in that era (mid 1970's - mid 1980 's ?



I’m not an expert on the 475, but I believe you are correct, starting in the 50 Volt lines.

Tant caps from the 1970’s & 1980’s had failures over time, due to impurities that crept in during manufacturing.

I tend to replace old 20V tants with 35V tants.

If Changing to electrolytic, triple the value… 22 uF/20Volt tant -> 100uF/25, 35, or 50Volt electrolytic



Steven


Tom Lee
 

I don’t fully agree with that recommendation, although it is fine in many circumstances. The impedance of tants stays low at high freqs, while aluminums generally don’t. Tripling the capacitance doesn’t necessarily reduce the impedance. Above self-resonance the impedance is dominated by parasitics, not the capacitive reactance.

Sent from my iThing, so please forgive brevity and typos


If Changing to electrolytic, triple the value… 22 uF/20Volt tant -> 100uF/25, 35, or 50Volt electrolytic

Steven


Mlynch001
 

I believe that you would be better off changing to a modern and higher voltage rated tantalum cap rather than substituting in a potentially inferior performing aluminum electrolytic cap. This will keep the circuits closer to the original design, but the higher voltage rated caps should give more margin of safety. My 2 cents and how I repair the scopes that come across the bench.

--
Michael Lynch
Dardanelle, AR


Redguuz
 

Michael,

I was indeed planning to substitute burnt Tantalums, by similar ones,albeit with a substantial higher working voltage.
This is a proven repair method in REVOX, STUDER audio equipment where Tantalum caps (Red Devils) consistently fail, because STUDER elected to choose 6V ones on 5V power rails: No margin whatsoever!

So far, my repairs of e.g. a REVOX A-700, hold on nicely (all tantalums on power rails replaced), but tantalum coupling capacitors left alone.

Question:
I contemplated to replace 1 uF and 2.2 uF tantalums on power rails by film caps (WIMA MKS2, https://www.wima.de/wp-content/uploads/media/e_WIMA_MKS_2.pdf ), because those are small enough to fit in the location of a tantalum cap.
Would these film caps have adequate specs to replace a tantalum equivalent?


Mark Vincent
 

Martin,

Replacing tantalums with film is fine. I have replaced a lot of the axial types that are 1mfd and lower with film. The radial types I also replaced with film. The originals were high ESR. One place that a film replaces a tantalum is the 2400 series. These have three 4,7mfd tantalums that go bad. I have not had any problems with a film replacement. Using film to replace lower value electrolytics works well. Places where I have done it is the crt bias filter, on the primary of the oscillator of the 7000 series power supplies (the 1 and 2mfd electrolytics) and other places. The Wimas you mentioned are great. I have used these. I have used the Kemet R82 series with the suffix DQ60J. These are the long lead types in 5% tolerance. Advantages of film are they are not polarized and will dry out. I measured a new 1mfd tantalum and film. The film was "0" (not literally/shorted) ohms on my meter while the tantalum was several ohms. I have an analogue ESR meter.

Using a higher value tantalum to replace a decoupling tantalum is fine. For example, using a 10-22mfd to replace a 6.8mfd. For areas where the value is for a frequency determining or filter, use the same value, higher voltage is fine. So far using a 25V tantalum on a 15V line has not failed. It is operating 60% of the rated voltage. Paralleling a tantalum with a low ESR electrolytic can be done to get lower ESR and higher capacitance.

Mark


Michael W. Lynch
 

Although I have never used film caps myself, I would think that they would be a good alternative to tantalum caps. Much better than aluminum electrolytics, for sure.

--
Michael Lynch
Dardanelle, AR


pdxareaid
 

coincidentally but apropos, on a 465B, i just had a film cap fail that was apparently used by a previous owner to replace a failed tantalum filter cap. he used the same voltage rating.
it was rated 20V in a 15V circuit. i will replace with either a tant or film of same capacitance but at a higher voltage rating.